1. Field of the Invention
This invention relates in general to mobile tanks for transporting liquid, and in particular to a vent valve for a mobile tank for reducing negative pressure in the tank during unloading.
2. Description of the Prior Art
Tank trucks for hauling bulk liquids normally have a discharge port and valve at the rear and bottom of the tank, and an intake port in the top. Liquids are poured into the intake, which is then sealed. At the destination, the liquid is drawn out the discharge valve and pumped to a storage facility. During unloading, it is essential that the tank be vented, otherwise the negative pressure, or vacuum, created by the draining liquid, may collapse the tank. To avoid this, the tanks have a vent port in the top. This port is normally covered by a cap during transport to avoid contamination and spillage. At the unloading site, the workers must be careful to remove the cap prior to withdrawl. Occasionally, this step is inadvertently omitted, and the tank collapses as a result.
Spring-loaded vent valves that automatically open under negative pressure are known. One disadvantage, however, is that the valve may freeze or lock in a closed position. This might occur particularly in a tank for hauling edible, viscous liquids, such as vegetable oil. Baffles are normally forbidden in these types of tanks, resulting in sloshing of the liquid. The sloshing liquid can cover the vent valve and congeal, preventing its opening, particularly if the spring protrudes downwardly. Cooler temperatures may freeze the liquid adjacent the vent valve, preventing its opening. Also, the sloshing itself may cause sufficient negative pressure to open the valve and result in spillage.